opt: constrain expression indexes with IS NULL expressions

pkg/sql/opt/xform/general_funcs.go

@@ -372,9 +372,7 @@ func (c *CustomFuncs) findConstantFilterCols(
 			}
 
 			datum := span.StartKey().Value(0)
-			if datum != tree.DNull {
-				constFilterCols[colID] = c.e.f.ConstructConstVal(datum, colTyp)
-			}
+			constFilterCols[colID] = c.e.f.ConstructConstVal(datum, colTyp)
 		}
 	}
 }

pkg/sql/opt/xform/select_funcs.go

@@ -415,7 +415,7 @@ func (c *CustomFuncs) GenerateConstrainedScans(
 
 	sb.Init(c, scanPrivate.Table)
 
-	// Check constraint and computed column filters
+	// Build optional filters from check constraint and computed column filters.
 	optionalFilters, filterColumns :=
 		c.GetOptionalFiltersAndFilterColumns(explicitFilters, scanPrivate)
 
@@ -424,7 +424,8 @@ func (c *CustomFuncs) GenerateConstrainedScans(
 	iter.Init(c.e.evalCtx, c.e.f, c.e.mem, &c.im, scanPrivate, explicitFilters, rejectInvertedIndexes)
 	iter.ForEach(func(index cat.Index, filters memo.FiltersExpr, indexCols opt.ColSet, isCovering bool, constProj memo.ProjectionsExpr) {
 
-		// A structure describing which index partitions are local to the gateway region
+		// Create a prefix sorter that describes which index partitions are
+		// local to the gateway region.
 		prefixSorter, _ := tabMeta.IndexPartitionLocality(scanPrivate.Index, index, c.e.evalCtx)
 
 		// Build Constraints to scan a subset of the table Spans.
@@ -491,7 +492,7 @@ func (c *CustomFuncs) GenerateConstrainedScans(
 // tryFoldComputedCol tries to reduce the computed column with the given column
 // ID into a constant value, by evaluating it with respect to a set of other
 // columns that are constant. If the computed column is constant, enter it into
-// the constCols map and return false. Otherwise, return false.
+// the constCols map and return true. Otherwise, return false.
 //
 func (c *CustomFuncs) tryFoldComputedCol(
 	tabMeta *opt.TableMeta, computedColID opt.ColumnID, constCols constColsMap,

pkg/sql/opt/xform/testdata/rules/computed

@@ -127,17 +127,14 @@ select
  └── filters
       └── (k_int:1 = 2) OR (k_int:1 = 3) [outer=(1), constraints=(/1: [/2 - /2] [/3 - /3]; tight)]
 
-# Don't constrain the index for a NULL value.
+# Constrain the index for a NULL value.
 opt
 SELECT k_int FROM t_int WHERE k_int IS NULL
 ----
-select
+scan t_int@c_int_index
  ├── columns: k_int:1
- ├── fd: ()-->(1)
- ├── scan t_int@c_int_index
- │    └── columns: k_int:1
- └── filters
-      └── k_int:1 IS NULL [outer=(1), constraints=(/1: [/NULL - /NULL]; tight), fd=()-->(1)]
+ ├── constraint: /2/1/4: [/NULL/NULL - /NULL/NULL]
+ └── fd: ()-->(1)
 
 
 # Don't constrain the index when the computed column has a volatile function.
@@ -267,3 +264,32 @@ project
       │         └── key: (1)
       └── filters
            └── d:4 = 1 [outer=(4), immutable, constraints=(/4: [/1 - /1]; tight), fd=()-->(4)]
+
+# Regression test for #83390. The optimizer should constrain an expression index
+# with an IS NULL expression.
+exec-ddl
+CREATE TABLE t83390 (
+  k INT PRIMARY KEY,
+  a INT,
+  INDEX idx ((a IS NULL))
+)
+----
+
+opt
+SELECT * FROM t83390@idx WHERE a IS NULL
+----
+select
+ ├── columns: k:1!null a:2
+ ├── key: (1)
+ ├── fd: ()-->(2)
+ ├── index-join t83390
+ │    ├── columns: k:1!null a:2
+ │    ├── key: (1)
+ │    ├── fd: (1)-->(2)
+ │    └── scan t83390@idx
+ │         ├── columns: k:1!null
+ │         ├── constraint: /5/1: [/true - /true]
+ │         ├── flags: force-index=idx
+ │         └── key: (1)
+ └── filters
+      └── a:2 IS NULL [outer=(2), constraints=(/2: [/NULL - /NULL]; tight), fd=()-->(2)]